Communication systems are designed to transfer information between two devices over a channel in the presence of disturbing influences. Intersymbol interference (ISI) is one well-known disturbing influence in which transmitted symbols become elongated and interfere with adjacently transmitted symbols. This spreading or “smearing” of symbols is generally caused by multipath propagation within those channels. Because ISI has the same effect as noise, communication is made less reliable.
One of the most basic solutions for mitigating the effects of ISI is slowing down the speed at which symbols are transmitted over a channel. More specifically, the transmission speed can be slowed down such that a symbol is only transmitted after allowing previously transmitted symbol pulses to dissipate. The time it takes for a transmitted symbol pulse to dissipate is called delay spread, whereas the original time of the symbol pulse (including any time before the next symbol pulse is transmitted) is called the symbol time. No ISI will occur if the delay spread is less than or equal to the symbol time. Although slowing down the symbol rate can eliminate or reduce the effects of ISI, it is generally an unacceptable solution for many of today's bandwidth intensive communication applications, such as those involving the transfer of multimedia content.
Orthogonal frequency division multiplexing (OFDM) is a multicarrier communication scheme that builds on this basic solution of slowing down the symbol rate to mitigate ISI. In an OFDM communication system, a plurality of orthogonal sub-carriers are transmitted over a single channel at the same time. The symbol rate of the communication system can be divided among the sub-carriers, allowing the symbol time to be increased and, thus, the effects of ISI to be reduced and more easily compensated for using equalization. Although OFDM provides a good solution for ISI mitigation, it suffers from several drawbacks, including a high peak-to-average power ratio (PAPR), which limits the efficiency of power amplifiers used in OFDM communication devices.
An alternative to OFDM that does not suffer from a high PAPR is single-carrier (SC) modulation combined with frequency-domain equalization (FDE). A SC communication device transmits and receives a SC modulated by symbols that each convey one or more bits of information. FDE is a filtering process that is used to flatten the frequency response of the channel over which the SC is transmitted to mitigate the effects of ISI. In general, FDE is performed on a block of symbols received via the SC and involves a fast Fourier transform (FFT) of the block of symbols and a channel inversion operation. More specifically, the FFT converts a time domain block of symbols received via the SC into a frequency domain signal, which is then equalized by multiplying it point-by-point by an estimate of the inverse frequency response of the channel. SC modulation combined with FDE is an effective technique for reducing the effects of ISI and delivers performance in line with OFDM, even for channels with long delay spread (i.e., channels with long impulse responses).
However, one drawback of SC communication devices that use FDE to mitigate ISI is that the structure used to perform FDE generally does not scale in terms of performance with changing channel conditions. As a result, FDE is generally always performed under the assumption of worst case channel conditions. Because multipath propagation of a channel often changes and is often below worst case conditions, FDE is often performed in SC communication devices at a level beyond what is required at the cost of excess power being consumed. For example, at any given point in time the multipath conditions of a channel may not be severe and may result in a short delay spread. Under these conditions, a SC communication device receiving information over the channel and using FDE to mitigate ISI may be performing and consuming power beyond what is required to properly recover the information. Since power consumption is often critical in SC communication devices that may be dependent on batteries, for example, any excess power consumed decreases the utility of these devices.
Another drawback of SC communication devices that use FDE to mitigate ISI is that the structure used to perform FDE generally does not scale in terms of performance with the signal-to-noise ratio (SNR) requirement of the receiver (i.e. with the SNR required or desired for properly recovering information modulated onto a SC), which is often dynamic. For example, the SNR requirement of the receiver can change based on the constellation order of the digital modulation scheme used to modulate the SC. If an SC communication device that uses FDE to mitigate ISI is currently receiving a SC modulated in accordance with a digital modulation scheme having a relatively low-order constellation (e.g., BPSK or QPSK), FDE likely will be performed in the SC communication device at a level beyond what is required at the cost of excess power being consumed.
Therefore, what is needed is an apparatus and method for reducing the power consumption of a SC communication device that uses FDE when multipath propagation conditions of the channel permit or when the dynamic SNR requirement of the SC communication device permits.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.